1. Field of the Invention
The present invention relates to a method for forming a deposition film such as a photoconductive film, a semiconductive film, an inorganic insulative film or an organic resin film by discharge under a reduced pressure.
2. Description of the Prior Art
In a method for forming a deposition film having a desired characteristic on a desired substrate by introducing a deposition film forming gas (source gas) into a reduced pressure chamber (deposition chamber) and depositing the film by utilizing a plasma phenomenon by discharge, it is inevitable that a certain directivity appears in a plasma reaction because of necessity of the application of an electric field in conducting the process. As a result, it is very difficult to maintain a uniform discharge intensity over a vicinity of the substrate and hence it is very difficult to form a deposition film having a uniform composition and a uniform thickness over the entire substrate.
For example, a technique for decomposing silane (SiH.sub.4) gas by discharging to form a hydrogenated amorphous silicon (a-Si:H) film on a heated substrate in order to utilize it in various photoelectric conversion devices or electronic devices has been reported. For the a-Si:H film having a structure-sensitive and semiconductive property, the ununiformity of the film composition sensitivity appears as variations in electrical, optical andf physical characteristics and hence it is a serious problem. When the a-Si:H film is used as an electrophotographic photoconductor, it is usually deposited to a high thickness over a large area of a cylindrical substrate. Accordingly, the problem is more serious. Since the film compositions and the film thicknesses of the films deposited at the periphery and center of the cylindrical substrate are usually non-uniform, a deterioration of an image, such as a defect or non-uniformity in density frequently occurs.